The present invention relates to hydrocarbon well logging systems; more particularly, it relates to a computer-based system for acquisition, presentation, processing and recording of nuclear hydrocarbon well logging data.
Well logging systems have been utilized in hydrocarbon exploration for many years. Well logging systems provide data for use by geologists and petroleum engineers in making many determinations pertinent to hydrocarbon exploration. In particular, well logging systems provide data for subsurface structural mapping, defining the lithology of subsurface formations, identifying hydrocarbon productive zones, and interpreting reservoir characteristics and content.
There are many types of well logging systems, each of which operates on the basis of some physical phenomenon. An induction logging system measures the conductivity of formations penetrated by a borehole. An acoustic well logging system measures the velocity at which a compressional wave traverses a formation immediately adjacent a borehole. A density logging system measures formation bulk density. A deviation logging system measures the magnitude and direction of dip of the formations encountered by a borehole.
In addition to the above well logging systems, there are also various types of nuclear well logging systems. These include gamma ray spectral logging systems which rely on the spectral analysis of natural and induced gamma rays. There is also a neutron absorption logging system which measures the decay rate of neutron population following a burst of neutrons into a formation from a pulsed neutron source. The Dresser Atlas Division of Dresser Industries, Inc. has offered nuclear well logging systems of these types under the trademarks SPECTRALOG, CARBON/OXYGEN LOG, and NEUTRON LIFETIME LOG.
Well logging methods involving the measurement of gamma rays, which are electromagnetic waves produced by unstable radioactive elements as their atoms undergo spontaneous or induced transformations, may be conducted in either cased or open boreholes. Spectral analysis of natural gamma rays is particularly useful in the identification of lithologies which could be potentially hydrocarbon productive. Also, spectral analysis of gamma rays is utilized in determining oil saturation in formations that contain a low or unknown salinity formation water. However, when fresh water is present, a spectral analysis of gamma rays resulting from neutron bombardment, i.e., induced gamma rays, provides an improved evaluation of formation lithology.
The three primary sources of natural radioactivity usually observed in reservoir rocks are thorium, uranium and potassium. Well logging involving spectral analysis of natural gamma rays provides a quantitative measurement of these elements. Both uranium and thorium are characterized by specific decay series. Potassium consists of three isotopes, of which the only unstable isotope is the nuclide potassium-40. Well logging systems providing spectral analysis of natural gamma rays measure the total gamma ray counts, the gamma rays emitted by potassium at 1.46 MeV, the uranium series nuclide bismuth emanating gamma rays at 1.764 Mev, and the thorium series nuclide thallium emanating gamma rays at 2.614 MeV.
Prior art well logging systems for conducting spectral analysis of natural gamma rays have included a subsurface well logging instrument to traverse a well borehole. The instrument includes a gamma spectrometer comprising a thallium-activated sodium iodide crystal optically coupled to a photomultiplier tube. A downhole electronic amplifier provides voltage amplification and transmits detector voltage pulse signals uphole through a logging cable to surface instrumentation. The surface instrumentation consists of an electronic amplifier, a multichannel analyzer, a digital panel, and a logging camera.
Pulse signals reaching the surface pass through an electronic amplifier to the multichannel analyzer and the digital panel. The multichannel analyzer provides for a total pulse count and selects pulses within prescribed energy windows for separate counting. The digital panel computes background radiation-corrected count rates from the raw logging data by means of a mathematical spectrum stripping technique. The energy windows of the multichannel analyzer are selected to correspond to the characteristic energies of potassium, uranium and thorium.
The digital panel includes four count rate meters (CRM). The counters accumulate the total number of gamma rays measured (total count rate, counts per minute) and the background corrected count rates in each of the multichannel analyzer energy windows for potassium, uranium and thorium. The outputs from each CRM, as a function of depth, are displayed by the logging camera on film. The logging film comprises four tracks. One track is the total counts in counts per minute. In adjacent tracks, a potassium curve is recorded by percentage, a uranium curve is recorded in parts per million, and a thorium curve is recorded in parts per million.
Well logging systems performing spectral analysis of induced gamma rays utilize a pulsed neutron source producing high energy neutrons. When carbon and oxygen are bombarded by high-energy neutrons, both emit gamma rays characteristic of the respective nuclei. The carbon gamma ray energy is 4.43 MeV and the predominant oxygen gamma ray is 6.13 MeV. The gamma rays are detected by a scintillation spectrometer, calibrated to count pulses in the energy ranges most indicative of carbon and oxygen. Information available in the spectrum analyzed includes measurements of the inelastic gamma rays of calcium and silicon. Also, after the inelastic reactions have ceased, measurements of the gamma rays of capture of silicon and calcium are made.
Neutrons of sufficient energy to excite a carbon or oxygen nucleus are found to exist in a subsurface formation for only a brief period of time. Accordingly, the detector is gated and synchronized to make a measurement while neutrons are being emitted from the source. A carbon/oxygen ratio is derived by taking a ratio of the gamma ray counts in the selected energy windows.
The gamma ray measurements are presented in a conventional well log format comprising continuous plotter tracks. One track is used to monitor the output of the neutron source. Adjacent tracks contain a carbon/oxygen ratio curve, a silicon/calcium ratio curve, and an inelastic calcium/silicon ratio curve.
Well logging systems for measuring neutron absorption in a formation uses a pulsed neutron source providing bursts of very fast, high-energy neutrons. Pulsing the neutron source permits the measurement of the macroscopic thermal neutron absorption capture cross-section .SIGMA. of a formation. The capture cross-section of a reservoir rock is indicative of the porosity, formation water salinity, and the quantity and type of hydrocarbons contained in the pore spaces.
Neutrons leaving the pulsed source interact with the surrounding materials and are slowed down. In a well logging environment, hydrogen in the surrounding water and hydrocarbons act to slow the neutrons. After the neutrons have been slowed to the thermal state, they are captured by atoms in the surrounding matter. Atoms capturing neutrons are in an excited state; and after a short time, gamma rays are emitted as the atom returns to a stable state.
The number of gamma rays present at any time is directly proportioned to the number of thermal neutrons, i.e., the thermal neutron population. The decay rate of this neutron population is an exponential function, and is defined by specifying the time required for the thermal neutron population to decrease to one-half. This time is referred to as a neutron "half-lifetime". While it is actually the neutron lifetime that is measured, the more useful parameter is the capture cross-section. Capture cross-section and neutron lifetime are inversely related, with capture cross-section being a measure of the rate at which thermal neutrons are captured in the formation. Analysis of formations in this manner is referred to as "neutron decay analysis".
The measurement of neutron population decay rate is made cyclically. The neutron source is pulsed for 20-30 microseconds to create a neutron population. Since neutron population decay is a time-related function, only two timereferenced gamma ray count measurements are necessary. The capture gamma rays are normally detected from time intervals that are 400-600 microseconds and 700-900 microseconds after each neutron burst. As the neutron source is pulsed and the measurements made, the subsurface well logging instrument is continuously pulled up the borehole.
The recorded log consists of four curves or tracks on a plotter. The capture gamma rays measured during the first measurement time period are recorded on one track. The capture gamma rays measured during the second measurement time period are recorded on a second track. On the third and fourth tracks, there are recorded a monitor of the neutron source output and the calculated capture cross-section. Capture cross-section is continuously calculated from the measurements made during the two measurement time periods.
Along with the thermal neutron log, an epithermal neutron log may be simultaneously recorded. Also, casing collars may be recorded.
The prior art nuclear well logging systems, though proving to be a very valuable tool in oil and gas exploration, have required the attention of skilled operators in order to produce consistent operation. Without skilled, experienced operators, giving full and complete attention to operating the well logging system, results are inconsistent, requiring logging runs to be repeated over and over until uniform well log data is obtained. Since the logging of a well requires a cessation of all other well site operations, valuable time is lost and a substantial cost is incurred where well logging is not quickly concluded. Accordingly, it is desirable for a well logging system, particularly a nuclear well logging system, to be capable of producing accurate, reliable well log data on a consistent basis. The present invention is directed to achieving this end by promoting consistent, reproducible well logging data acquisition with less dependence upon interaction of the logging operation with an experienced operator.